ABSTRACT One of the greatest challenges in human genetics is to understand the genetic basis of complex disorders, such as congenital heart disease (CHD). Studies of rare diseases with known genetic etiologies such as the 22q11.2 deletion syndrome (DiGeorge/velo-cardio-facial syndrome), can pinpoint disease genes and mechanisms. Approximately 65% of 22q11DS patients have a congenital heart defect, mostly affecting the cardiac outflow tract (OFT). These range from mild to severe and include persistent truncus arteriosus and tetralogy of Fallot, at the severe end. Although TBX1, encoding a T-box transcription factor has been the strongest candidate gene for CHD on chromosome 22q11.2, patients with similar heart defects, but non- overlapping deletions within this region have been identified. In these patients with atypical deletions, CRKL, encoding a cytoplasmic adaptor to receptor tyrosine kinase signaling is deleted, but not TBX1. This implicates CRKL as a true disease gene. Although there have been extensive studies of TBX1, CRKL is understudied with respect to heart development. The neural crest cells are a critical cell population in the pharyngeal apparatus needed for proper alignment of the cardiac OFT. We propose to explore neural crest cell functions of Crkl by evaluating new upstream and downstream pathways in Aim 1. We will then delineate the subset that is critical for OFT development as mediated by Tbx1 and Crkl. Crkl-/- mice die at E16.5, earlier than can be expected based upon only having OFT defects. Our pathological investigation has found that Crkl-/- embryos have OFT valve defects in addition to alignment malformations. OFT valve anomalies are highly relevant to 22q11DS, as 9% of patients have these defects. This is a newly recognized defect for which the molecular mechanisms are not known. Based upon this, and the presence of similar defects in endocardial-specific null mutant embryos, we hypothesize that Crkl is required in this tissue to form derivative OFT valves via VEGF- MAPK and additional signaling pathways. Endocardial functions will be tested in Aim 2. This work is also clinically important because middle-aged 22q11DS patients have now been reported to have sudden unexplained death, unconnected to the well-known intracardiac malformations. This might be related to the presence of aortic valve stenosis, which accounts for 6.6% of 22q11.2 deletion patients. To connect the mouse work in Aims 1 and 2 back to the human patients, we will evaluate existing whole exome sequence (WES) from the current largest cohort that we are aware, of >1,000, 22q11DS patients for mutations in CRKL interaction network genes in Aim 3. Exploratory studies of non-coding sequences in the candidate genes will be performed. Validation will be done using more DNA samples from 500 22q11DS subjects and from emerging WES data on non-syndromic CHD patients. The synergy of mouse and human studies will offer rapid discoveries for a deep understanding of the complex genetic etiology of CHD.